This trick works with Java I2P, it would be great if it works with i2pd too.
Point here is setting outproxy to local proxy, for example 127.0.0.1:9050. All i2p traffic goes through i2p, all clearnet/onion traffic goes through Tor.
With that we can easily build I2P+Tor browser bundle on top of modified TorBrowser. Just modify "Browser/TorBrowser/Data/Tor/torrc-defaults" to listen on 9153, set that address as outproxy and run i2pd socks proxy at 9150. TorBrowser displays like browser isn't working through Tor, but actually it works fine (so some UI modifications should be done too).
Who use both i2pd and TorBrowser, please try if it works!